UHF Antennas Direct 91XG (aka XG91) 23-Director Corner-FD-Yagi analyzed using 4nec2. [Folded Dipole, 1-ea Director-Rod, 22-ea Bowtie Directors (ALL SAME SIZE) & a Corner Reflector.] Boom Length=90.2-in.
RevA: UHF Raw Gain = 11.5 dbi (470 MHz) to 16.9 dBi (698 MHz) to 17.8 dbi (758 MHz), F/R & F/B Ratio Min = 19.9 dB (Excellent) and SWR (300-ohms) Under 3.0 (Fair).
RevF: UHF Raw Gain = 11.3 dbi (470 MHz) to 16.6 dBi (698 MHz) to 17.5 dbi (758 MHz), F/R & F/B Ratio Min = 19.8 dB (Excellent) and SWR (300-ohms) Under 2.3 (Excellent).
Hi-VHF Raw Gain plots show a reasonable amount of Gain, with more toward the REAR. SWR is very excessive, which may or may not cause problems, esp. on weak channels. However, a couple years ago A-D changed to a PCB Balun (vice usual Hybrid Transformer) which has very high loss on VHF frequencies....so Hi-VHF charts only apply to old versions.
XG91a.nec file was converted (by 300ohm) from Ken Nist's EZNEC file: http://www.wuala.com/300ohm/Documents 300ohm also posted a 4nec2 file for a 91XG with SIX additional Directors. Boom Length=122.1-in.
I added some FR and RP statements and a variable SOURCE wire radius to adjust AGT=1.0 for both UHF and Hi-VHF (XG91a_RevA.nec). The latest, XG91a_RevF.nec, removed some extraneous wires from the Directors, eliminating "Sharp Angle" Warnings and changed SOURCE Wire so AGT=1.0 and NO Errors or Warnings.
NOTE: There are several variations of the XG91/91XG from different manufacturers. Carefully check your dimensions against Ken Nist's model...esp. if you think you have a 75-ohm balun.
EDIT (14Nov2012): All Elevation (Vertical Plane) Charts were mislabeled as "Azimuthal". Chart labels corrected and added Azimuthal Chart set. Added Rev E 4nec2 version with AGT=1.0 and NO Errors and Only Warnings when Run AGT and NONE when Run 4nec2 file or Segment Test (weird).
EDIT (8Jan2012): Incorrect RevC 4nec2 file had erroneously been uploaded (was RevD). Uploaded correct RevC 4nec2 file.
EDIT (22Aug2013): Uploaded RevF 4nec2 file which eliminated ALL Warnings. Note minor Raw Gain differences (lower than A-D's published modeling results) and SWR Max = 2.3 (better agreement with A-D's modeling results): http://www.antennasdirect.com/cmss_files/attachmentlibrary/Technical%20Data%20PDF%27s/91XG-TDS.pdf
enlarge 160KB, 791x1024 33 A-D 91XG - RevC 4nec2 File No Errors and only a few Warnings re SOURCE Wire being very small. Residual AGT correction = 0.35 dB (Subtract from Gain).
enlarge 0KB, 1024x263 34 A-D 91XG - RevA Top View - 90.2-in Boom [One small box = 1-inch]
enlarge 158KB, 791x1024 35 A-D 91XG - RevE 4nec2 File No Errors. AGT=1.0. Warnings ONLY when Run AGT.
enlarge 158KB, 791x1024 36 A-D 91XG - RevF 4nec2 File No Errors or Warnings. AGT=1.0.
enlarge 122KB, 1016x1024 37 A-D 91XG - RevF 3D View
enlarge 279KB, 869x1024 38 A-D 91XG - RevF Front View [1 large square = 2.5 inches]
enlarge 124KB, 1152x468 39 A-D 91XG - RevF Top View [1 large square = 12.5 inches]
enlarge 152KB, 1024x677 46 A-D 91XG - RevF Ch2-13 Raw Gain - More to REAR NOTE: Typical for Older 91XG's. Newer 91XG's use Printed Circuit Board (PCB) Balun which attenuates VHF Frequencies.
